Disclination morphology of nematic and smectic liquid crystals in planar twist cells

We employ optical microscopy, with and without an external magnetic field, to characterize the three-dimensional (3D) morphology and line tension of disclinations in nematic and smectic liquid crystals (LCs) in planar twist cells. In the nematic LC, twist disclinations are generated by quenching the LC (5CB) into the nematic phase; the disclinations form loops separating domains of opposite twist handedness. We investigate the free disclination segments that penetrate into the bulk nematic. Confocal microscopy is utilized to measure the profiles of these free disclinations and test theoretical predictions about their shape, and then applied magnetic fields are utilized to deform the free disclinations into circular arcs whose curvature depends on the field-induced energy difference between opposite twist domains. The line tension of the disclinations is obtained from an energy-balance equation that relates disclination curvature to magnetic field. Using a different LC (8CB), we cool the sample from its nematic phase into its smectic phase in these same cells. We find that large domain of smectic phase forms in the middle of the cell, sandwiched between two twisting nematic layers located near the sample cell substrates. During this process, fluorescence, polarization, and confocal microscopies reveal that the twist disclination in the nematic phase evolves into contours with remarkable 3D morphology shaped by focal conic domains that arise in the smectic phase between domains with different twist.